I'm lost with this one also.
I'm a pro Photographer and I sell a lot of my own images but as for playing with other peoples, ie Photoshop enhancement or total re working to produce an amazing image, I'm sad to say that their isn't much call for it.
Many people now have photo manipulation software and even Photoshop cs3 / cs4 dowloaded from the internet and know how to use it, plus products like Picasa although basic, allow for manipulation and even printing and dlivery to your door!
You also need to cosider that many images taken by customers with a basic digital camera may be far too small in both size and quality for you to manipulate well. You may also have a problem if someone using a pro or semi pro camera mails (or tries to) a very large file ie 5-10mb + and they find that their e-mail program won't allow it to be sent as it is too big.
I'm not trying to put you off but you really will need to do a little market research first to see if it will be viable technicaly as well as financialy .
If you do go ahead with it (and I can only presume that is what you are sking from your post), then I wish you the very best of luck.
Regards
Marc